Power devices such as a liquid crystal displays (LCDs) and driver ICs require both low voltage (LV) operation for driving an associated logic circuit and high voltage (HV) operation for driving the LCD during operation. A double diffused drain MOS (DDDMOS) is a popular HV structure for devices having an operating voltage less than about 20 volts. Driving current (Idsat) is an important performance parameter for HV devices. It is desirable to have a high and stable driving current, particularly for high-speed or high frequency applications.
Illustrated in FIG. 1 is a conventional HVMOS transistor, which includes a substrate 1 wherein field oxide regions 2 have been formed. The transistor includes a source 3 that comprises an N+ or P+ (N+/P+) region 4. The drain 5 of the transistor comprises a first diffusion region 6 and a second diffusion region 7, as shown. The first diffusion region 6 may comprise an NDD or PDD (NDD/PDD) region comprising an n/p type double diffused region, for example. A lightly doped NLDD or PLDD (NLDD/PLDD) region comprising an n/p type lightly doped double diffused region 8 may also be formed in the source 3, such as shown. A channel region 9 resides between the source 3 and the drain 5, beneath the gate oxide 10 and the gate 11. Spacers 12 are formed on either side of the gate electrode 11.
A problem with the HVMOS transistor shown in FIG. 1 is that the etching process used to form the spacers 12 has bad selectivity. Therefore, the etching forms a surface damage region 15 within the substrate 1. Since the NDD/PDD region 6 is a lightly doped region, surface damage region 15 causes Idsat to drop. Also, the spacer-etching rate has ring type performance, the etching rate at the wafer center is different compared to that of the wafer edge, and so it makes the Idsat value low and its uniformity very poor. Poor etch selectivity also causes oxide depletion 13 from the isolation regions 2 and poly damage 16 at the gate electrode 11. Conventional methods and structures may therefore suffer from performance degradation and reduced reliability.
What is needed in the art are improved methods of forming double diffused drain structures and improved double diffused drain structures, particularly in relation to HV transistors.